The present invention relates to measurement apparatus and, more particularly, to measurement apparatus in which a measured quantity can be characterized by a frequency of an alternating signal.
Apparatus for generating pulses having a frequency related to a measured quantity are in routine use. For example, certain electric meters contain pulse initiators which produce output pulses at a frequency having a desired relationship to the power usage being measured. An output pulse may be generated, for example, in response to the consumption of 1 KWh, or any other desired quantity. Such output pulses are useable both by the utility and by the energy consumer for monitoring, recording or communicating the energy usage at substantially shorter intervals than is normally possible in the routine monthly reading of an electric meter. When combined with a clock, such pulses can be the basis of a time-of-use or demand metering system in which different tariffs are applied depending on the time of day and/or day of the week during which the consumption occurs or on the energy usage rate.
A conventional electric meter employs a rotor disk, typically of aluminum, driven at a speed proportional to the energy usage by rotating magnetic fields generated by voltage and current coils in the load circuit. The disk shaft is geared to a mechanical register for integrating energy usage and optionally energy demand over specified demand intervals. In addition, the disk shaft may be geared to a mechanical cam, photo-optical sensor or other rotation-sensing device, for mechanically or electromechanically opening and closing the contacts of a relay. The relay contact closures are made available on external connections.
The gear ratio between the disk shaft and the rotation-sensing device determines the relationship between the disk rotation frequency and the output pulse frequency. Utilities usually desire that the significance of the output pulses have one of a substantial number of different quantities. For example, utilities may wish to choose between 1, 10, 100 or 1000 KWh, or other quantity per pulse. As is well known, an electric meter may be required to measure power usage at any one of several different voltages over a number of different load current ranges. Furthermore, the relationships between load current and voltage, and the current and voltage applied to the electric meter, may be modified by current and voltage transformers interposed between the load circuit and the meter. Each combination of the above variables requires a different gear ratio for driving the cam. Each different gear ratio requires a different mechanical design. In order to supply a reasonable number of customer requirements, a stock of up to several hundred gear assemblies may be required. This represents a significant cost in designing, manufacturing in short runs, stocking, cataloging and administration. In addition, since each gear ratio represents a unique design, once a particular gear ratio is installed in a meter during manufacture, no simple way is available to change the ratio in the factory or the field without major disassembly and reassembly of the meter. As a consequence, custom manufacture, with its increased costs and lengthened delivery times, must be employed to meet many customer requirements.
In the ideal, pulse initiator portions in all meters would be identical and the ratios would be defined in a simple and inexpensive operation after manufacture, either prior to shipping or after receipt by the utility. Also, it is most desirable that provision be included for adding the pulse initiator to a meter not previously containing one and that the pulse input-output ratio can be defined in the added pulse initiator.
In practice, an electro-mechanical electric meter can experience reverse rotation of the rotor disk due to drift under the influence of stray magnetic fields at no load, or due to reverse flow of current. The drift is usually arrested within one turn by an anti-creep hole purposely formed in the disk to lock the disk in a fixed rotational position until dislodged therefrom by a substantial torque attendant to the resumption of energy usage by the load. During the reverse disk rotation, in order to reach the point at which the anti-creep hole restrains further reverse rotation, a conventional pulse initiator may generate one or more spurious pulses. Such spurious pulse generation is avoided by a mechanical ratchet device which prevents reverse disk rotation or reverse rotation of the pulse-initiator cam. Such a mechanical ratchet device may add an undesired frictional retarding force during forward operation.
Reverse flow of current may occur, for example, in an intertie between two power grids wherein one of the two grids is at times a supplier, and at other times a receiver, of power over the same circuit. In such a circumstance, the reverse rotation of the meter disk has economic significance which should not be ignored. One way of separately capturing the bi-directional power flow may include the use of two separate meters, each having a mechanical ratchet to prevent reverse rotation. One of the two meters is connected for forward rotation by power flow in one direction and the other is connected for forward rotation by power flow in the opposite direction. A further source of reverse flow may result from tampering by removal and reversal of a meter for a portion of the interval between meter readings. Such reversal can decrement a meter or, if reverse rotation is prevented by a mechanical detent, it can prevent the proper incrementing of the meter and the generation of pulses by the pulse initiator. The prior art provides examples of inversion detection devices which, upon detecting the reversal of a meter, automatically reverse the direction of meter movement with respect to current flow thereby maintaining unidirectional register integration regardless of meter reversal. Such inversion detection devices add to the cost of manufacture.
An electronic pulse scaler performing as a pulse initiator for producing an output related to electric power consumption preferably takes the preceding factors governing reverse rotation into account.
Non-contact devices for sensing rotation of a shaft are disclosed in, for example, U.S. Pat. Nos. 3,943,498; 4,034,292 and 4,321,531 wherein optical sensors produce two phase-displaced signals in response to the passage therepast of reflective optical markings or apertures rotating with the meter disk. The relative phasing of the two signals is interpretable to distinguish between forward and reverse rotation of the disk. An improved optical detection sensing device is also disclosed in co-pending U.S. patent applications Ser. Nos. 550,142, now abandoned and 550,407, now abandoned. The disclosures of the referenced patents and application are herein incorporated by reference. The availability of data in electronic form regarding disk rotation simplifies the application of an electronic pulse scaler to produce output pulses related to the disk rotation by a predetermined factor. The recited patents, although they disclose electronic means for separating forward from reverse rotation, fail to contain disclosure of a scaling function except, in the case of the U.S. Pat. No. 3,943,498 patent, a direct submultiple division. Such a direct submultiple division is incapable of providing the very wide range of ratios required to serve virtually all the needs of utility customers.
Although the electro-mechanical electric meter has been developed into a reliable, durable and precise measurement instrument, an all-electronic meter, including both the measurement and the integration functions may become technically or economically desirable. An all-electronic meter measures the values of voltage and current using any one of several conventional electronic techniques, and electronically multiplies the voltage and current to produce an output having a characteristic which is variable in proportion to the product i.e. to the power. An electronic register integrates the resulting output which thereupon may be displayed or transmitted in a manner analogous to conventional electro-mechanical or hybrid electro-mechanical/electronic meters. One suitable output of the product of the current and voltage includes a pulse train whose frequency is variable in proportion to the product. The integration of usage then requires only the counting of pulses over a given period of time to derive the total power usage during the period.
Practical manufacturing of an electronic meter requires the use of electronic components which have tolerances varying from the exact values specified during the design of the product. Such electronic meters conventionally include adjustable elements, most commonly variable resistances, which are adjusted during final assembly of the electric meter to overcome errors due to component tolerances. Such adjustable elements are more prone to changes in value during use than are fixed-value components. Thus, the inclusion of adjustable elements for final calibration of an electronic meter sets the stage for later increases in the probability of error or failure after a period of service. Furthermore, the vernier adjustment of such adjustable elements is relatively difficult to automate during final assembly of the electric meter. Finally, the range of adjustment feasibly available with adjustable elements falls far short of the range necessary to accommodate the different voltage and current combinations employed by the utilities. As a consequence, the need remains to manufacture and stock a substantial number of different meters and/or voltage and current transformers to satisfy the needs of the buyers of all-electronic electric meters.